Handle config not being set for synapse plugin modules

Some modules don't need any config, so having to define a `config` property
just to keep the loader happy is a bit annoying.

synapse/util/module_loader.py

@@ -34,7 +34,7 @@ def load_module(provider):
     provider_class = getattr(module, clz)
 
     try:
-        provider_config = provider_class.parse_config(provider["config"])
+        provider_config = provider_class.parse_config(provider.get("config"))
     except Exception as e:
         raise ConfigError("Failed to parse config for %r: %r" % (provider["module"], e))